Seventeen day old fetal rat cerebellum was cultured as aggregates for 3 weeks. Aggregates were isolated periodically during this period and analyzed for DNA, RNA, cerebroside and proteolipid protein. These results were compared with comparable data from in vivo cerebellum of the same post-conceptional age. Aggregates isolated 1 day after the start of the culture contained slightly more DNA than the inoculum which suggested the possible occurrence of cell division in the culture with 48 percent of day 1 DNA being present after 3 weeks. RNA/DNA ratios were below unity initially and increased about 50 percent during culture. These ratios decreased in in vivo tissue to 0.6 due to the proliferation of small granule cells during the postnatal period. Cerebroside content per mg DNA increased 3.5 fold in the aggregates compared to 5 fold in vivo when examined at the same post-conceptional age. Proteolipid protein measured on a comparable basis increased 2.5 fold in vitro and 4.5 fold in vivo.